A perpetual inventory system updates your stock count and cost of goods sold continuously, at every sale, return, and receipt. A periodic system waits: you count physically at intervals (monthly, quarterly, yearly) and reconstruct what must have happened in between. Both are legitimate accounting methods; only one of them can tell you what is on the shelf right now, and that difference decides which one an online seller can actually live with.
How each works, concretely
Periodic: you start the quarter with $30,000 of inventory at cost, purchase $45,000 during it, and count $25,000 at the end. COGS = 30,000 + 45,000 - 25,000 = $50,000. Simple, cheap, and entirely blind between counts, any theft, breakage, or miscount hides inside that COGS number, and your day-to-day stock figure is an estimate.
Perpetual: every transaction posts immediately, a sale decrements stock and books its cost, a receipt increments. The ledger always states current stock and running COGS. Physical counts still happen (cycle counting beats annual freezes), but they verify the system rather than create the numbers.
The trade, honestly
Periodic’s virtue is simplicity: no software requirement, minimal process, fine for a tiny catalog sold slowly in one place. Its costs are blindness and archaeology, you learn about problems at count time, weeks after they started, and shrinkage is indistinguishable from cost of sales.
Perpetual’s virtue is truth on demand: reorder decisions, stockout forecasts, and channel updates all need a live number. Its cost used to be significant (systems, scanners, discipline); for online sellers it has collapsed to roughly zero, because the platforms you already sell on are transaction-recording machines.
Why multichannel selling settles the question
The moment you sell the same stock pool on two channels, periodic stops being an option in any practical sense:
- Channels need live numbers to stop overselling. eBay and Amazon do not accept “we count quarterly” as an availability answer; they list whatever number they last heard. Stale numbers are the oversell mechanism, and a periodic system is stale by design.
- Combined velocity drives every operational decision: days of cover, reorder points, and buffer sizing all consume continuous per-SKU sales data that only a perpetual record produces.
- The perpetual system already exists in your stack. A hub that syncs your store and marketplaces, decrementing one shared pool at every sale anywhere and updating every channel in seconds, IS a perpetual inventory system for units. Unifystock plays exactly that role across Amazon, eBay, Shopify, WooCommerce, Etsy, and OpenCart, with the audit trail (per-channel sync log) that makes the record verifiable.
The accounting layer (COGS valuation, FIFO or weighted average) then reads from a truthful unit record instead of a quarterly reconstruction.
Common questions
Is periodic inventory ever the right choice for ecommerce?
At very small scale, one channel, slow turnover, a handful of SKUs, it is defensible. The switch point is the second channel or the first time a stockout surprises you.
Does perpetual inventory eliminate physical counts?
No, it changes their job. Counts become verification (cycle counts on a rotation) rather than the source of truth, and discrepancies become signals about process instead of quarterly mysteries.
Which method do accountants prefer?
Accountants care that the method is consistent and the records support it. Perpetual gives them cleaner interim statements; ask yours before changing methods mid-year, valuation method changes have tax implications.
Do I need a warehouse system for perpetual inventory?
No. WMS-grade tooling (bins, waves, scanning) is for warehouse operations. Unit-level perpetual truth across channels is what a sync platform provides without the warehouse apparatus.
Perpetual, without the project
Connect your channels and the perpetual record builds itself: every sale posts everywhere in seconds, with a log to prove it. From $49/month with unlimited orders, see pricing.